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Contents

Chapter XXIX. The Entodermal Canal

The first stages of the entodermal canal are described in Chapters IV. and IV., its earliest differentiation as the archenteron in Chapter XII. We have now to take up the differentiation of the various entodermal organs after the formation of the gill-clefts.

For convenience I prefix a list of all the organs or parts derived from the entodermal canal. They are :

Gill-clefts. 10. Duodenum.

Pharynx and tonsils.

Thyroid gland.

Thymus gland.

Larynx, trachea, and lungs.

Stomach.

Liver.

Pancreas. *

Rectum.

Yolk-sac.

Small intestines.

Caecum.

Vermix.

Colon.

Oesophagus.

Allantois.

Schwanzdarm.

Of these there have been already described — 1, the gill-clefts; 11, the yolk-sac; 17, the allantois, and, 18, the Schwanzdarm.

In this chapter is presented, firsts the history of the alimentary tract; second^ the history of the respiratory organs {i. e., of the above list, 5, larynx, trachea, and lungs).

I. The Alimentary Tract

Pharynx or Branchial Region. — That part of the archenteron in which the gill-clefts are situated becomes the pharynx of the adult. The entodermal pouches of the gill-clefts undergo profound modifications. The pouch of the first or hyo-mandibular cleft becomes the tubo-tjTnpanal canal, compare p. 738. The pouch of the second cleft becomes broad and shallow, and gives rise to the tonsils, p. 745. The remaining pouches, so far as I know, have no recognizable traces on the surface of the adult pharynx, though their epithelial walls are concerned in the development of the thyroid and thymus glands.

The pharyngeal cavity early becomes continuous with the mouth cavity by the rupture of the oral plate, p. 262.

The change of shape in the pharynx has never been traced, nor have Ave any definite knowledge of the histological development of its walls. In the adult it resembles the oesophagus histologically.

Its posterior limit is marked by the opening of the trachea. It is, therefore, a relatively small tract in the adult, although in the embryo, when the gill-clefts arise, it constitutes nearly half the archenteron. So too, among vertebrates, as we ascend the series, we find that the relative importance of the pharynx diminishee.

From the floor of the pharynx are developetl the tongue and the epiglottis; the tongue is treated, p. 592, in connection with the mouth-cavity ; the epiglottis is trejited, p. 778, in connection ^rith the la^J^lx.

Cervical Sinus (Sinus prce-cervicalis o/ ///«).— Although the cervical siniia is an ectodermal structure, yet its formation is due to mo<lifi cations of the gill-arches, and therefore ite historj- may Ix^

E resented conveniently in coimection with that of the pharynx. E. *ursy, 69. 1, H'i, gives the earliest accurate description of the cervical sinus known to me. He observed that in a cow's embiyo of 11 mm. the third and fourth branchial arches are much smaller than the others and constitute a triangular area depressed below the level of the surrounding external surface; the apex of the triangle points toward the ventral side. The corresponding stage in man is found in embryos of 9-10 mm., compare Fig. 219, cs. By the growth of the caudal margin of the second branchial arch the depressed aren becomes further invaginated ; Dursy compared the second arch to the operculum {Keimenaeckel) of Bshes — n comparison originally suggested byH. RathkeinlSa.'i, 25.1. His (" Anat. Menschlicher Einbryonen,'" III. 2S,) also, 86.3, has traced the invagination of the third and fourth gill-arches in the human embno, resulting in the formation of a deep fissure on each '"^ ""^"^ sido of the nefk somewhat t()ward

the ventral surface, Fig. i'i'i; owing to its jxffiition toward the ventral side His namctl the fissure prte-cervical sinus ; Riibl mistcx)k r i«,j#*w^^7^^^ the prefix tomeauheadwardof the

jf^ ffi__\,__ ^n neck, and accordingly made jin

•% y acrimonious attack, 86.1, uikju

His for saying that the sinus was A^ not connected with the neck,

".'^ Riibl's blunder was corrected bv

His, 86.3, 428. His has shown that the fourth arch is tiuned in Lu first, and that the third arch is turned in a little hiter ; the sinus Ih so narrow that the arches ct>ine in Fo 1. nof hePioDTiB™i ™ntact with the Opposite wall; the

Ri-B 11 Hu t oof Ri»' ectoderm of the arches concresces

^ h '^ « "" Ta" '^ I " "nt; with that of the caudal side of the

j[j , ' j"^ "" "' areh. ghius, the o])ening of whicli is thus

obliterated. The sinus is now an ep tl lial c rl c n nee ted with the epidermis on the one hand, anl oi the ther with two spaces lined with ectodenn: one space Cf ri-euiwnds t the e< t dermal furrow of the second gill-cleft, the othei t ) the cct derm il furrow of the third gill-cleft. All trace of tl e stcond fii r \ s soon obliterate*! (compare Fig. 434), but the rtmn int f th third f irrow perwifits longer and lies in dose pniximit\ t( the anla^ f tl t, th\"mus, Fig. 4U4. His, "Anat. mensclil.

Embrj-oDen," III., 104, regarded the buried remnant of the third ectodermal branchial furrow as the anlage of the thymuB. In 1886, he atill adhered to this opinion in an article, 86.3, which gives the fullest history of the sinus we have yet, but after the entodermal origin of the thymus had been demonstrated in various types, His reworked the question, and in a brief paper, 89,2, withdrew his earlier opinion.

So far as known, the cervical sinus entirely disappears, but its abnormal persistence may account for certain cysts occurring pathologically in the neck.

Tonsils. — The tonsils are developed from the second gill-cleft. In an embryo of four or five months, the shallow pouch which represents this cleft is found bounded in front by the arcus palalogloeeue, which is a survival of part of the second branchial arch, and is partly covered by the uvula, which is continued on to the wall of the pharynx !is a fold, the plica triangularis of His (" Anat. menschlicher Embryonen," Heft III., 82), which bounds the pouch on the dorsal side. The pouch is lined by the mucous membrane (entoderm plus ineseijchyma) of the pharynx.

The histogenesis of the tonsils has been made the subject of a long memoir by E. Retterer, 88.1, who maintains that the epithelium commingles with the connective tissue, forming a special angiothelial tissue of double origin. P. Stohr, 9 1 , 1 , and Gulland (Lab. Kept. E. Coll. Phy8.Edinburgh,IIl.,18'Jl) have shown that Retterer's view is erroneous. According to Stohr, the tonsil has, at three months, a stratified epithelium resting on mesenchyma without leucocytes. At four months the tonsillar fissures begin to branch, and the epithelium presents buds, some of which are the solid anlages of glands, while others are the commencements of branches of the tonsils. The formation of solid tonsillar buds continues not only througli the embryonic period, but also for a year afterbirth. The solid buds gradually become hollow by a change in the central cells, which assume a corneous ! appearance and gradually contract into a mass in the centre of the bud. Fig. 43:J, (?. Meanwhile, the cavity of the tonsil extends into the upper part of the bud, until it communicates with the space containing the degenerate<l mass, which is then expelled. The epithelium is at all periods sei»irated f rom the mesoderm by a distinct endothelial basement membrane, Fig. 4.33, h.ni, nevertheless it is penetrated by leucocj'tes during the fourth month. Fig. 433, / /. Up to the time of birth the uuinl)er of the immigrant cells in the epithelium gradually increases ; indeed they may become so numerous that the epithelium and l>asement membrane are scarcely recognizable. In the mesenchyma there are connect ive- tissue fibrillie at three months, and at

lo. 433._From a Seel a Human Embryo o

that stage there are also leucocytes scattered about, but the infiltration is diffuse. As the number of leucocytes increases, they show an increasing tendency to form groups — the anlages of follicles — but it is not until after birth that the follicles become well defined with distinct germinating centres. The leucocytes are probably derived from the blood by migration to the walls of the blood-vessels in loco. The Thymus is developed from the entoderm of the third gillcleft, as a thickening, which remains after the cleft aborts. That the thymus is of exclusively entodermal origin in all birds and mammals is extremely probable, though not quite certain. Fn:)riep, 91.2, 64, asserts that in sharks the thickening is identical with that which forms the epibranchial organ, a view that interprets the thymus as ectodermal. The form of the third gill-cleft in young embryos is described p. 264. In a human embryo from the beginning of the fifth week, His, 89.2, found the entodermal pouch of the third gill-cleft open. Fig. 434, 3; the entoderm in the distal part of the ck»ft is somewhat thickened, and is in immediate contact with the ectoderm of the cervical sinus. In a pig embryo of 11 mm. Born, 83.1, 288, finds the lower part of the entodermal pouch still open, but in the dorsal apex the epithelium has grown and obliterated the cavity. In a pig embryo of 13 mm. Bom, p. 29, finds the dorsal and distal end of the third pouch enlarged, and the rest transformed into a very narrow canal by which the end is connected with the pharynx proper. In a cow embr\'o of 12 mm., Froriep, 86.1, 23, found a verv" similar condition, but the lumen of the canal was beginning to disappear. In the rabbit at thirteen days, Piersol, 88. 1, 175, and Fig. 24, finds the distal dorsal dilatation of the |XMich very marked ; its walls are greatly thickenetl, but the central cavity still persists; the canal to the pharynx has become a solid epithelial cord. The connection of the |>ouch with the pharynx is S(X>n li>st, and the third entodermal gill-pouch may he then designate as the independent anlage of the thymus. This anlage is an elongated sac with thickened epithelial walls ; it occupies an oblique dorso- ventral line; its dorsal end is especially enlarged and corresponds to the future head of the thymus. Bom, p. 297, found the connection of the thymus with the pharynx severed in a pig embryo of 20 mm. F. P. Mall, 87. 1, 16-28, has followed the development of the thymus in the chick, and found it essentially identical with that in mammals ; the thickening of the entodermal walls begins the fourth day ; the fifth day the thymus separates from the pharjTix and becomes an elongated body, situated at about the same level as, and nearly parallel with, the pharynx and overlying the third and fourth aortic arches. The manner in which the thymus changes its form and position is clear from the reconstructions in Fig. 436, thm^ and therefore requires no special description.

The lumen of the anlage, though long persistent, is gradually obliterated until it completely disappears (pig of 35 mm.); in a pig embr\'o of 25 mm. the ventral end of the thymus is developing lateral buds, and in an embryo of 35 mm. the whole organ is budding (Born, 83.1, 306). A similar condition is found in the rabbit at sixteen days, in man about the twelfth week (KoUiker, " Grundriss," 2teAufl., 370, 371).

Histogenesis. — Kolliker ("Entwickelimgsgesch.," 2te Aufl., 878) records for the rabbit, that between the twentieth and twenty-third days the cells of the thymus become smaller and their outlines disappear, so that the organ appears to be an accumulation of small round nuclei. At about the same period blood-vessels and connective tissue grow into the epithelial anlage. After the penetration of the vessels the differentiation of the cortex and medulla is recognizable; in carmine preparations the cortex is the darker part. According to Stieda, 81.1, the concentric bodies of the adult thymus are derived from the epithelium (entoderm) .

The remarkable changes in the thymus after birth are outlined in all the principal " Anatomies. " For details see especially Af anassiew, 77.1.

Historical Note. — L. Stieda, 81.1, discovered in 1881 that the thymus gland arises in intimate connection with a gill-cleft. Kolliker in 1884 recorded (**Grundri8s," 2te Aufl., 369) that the primitive anlage of the gland was an epithelial mass. G. Born in an essay of great excellence, 83.1, demonstrated that the gland is developed from the entodermal lining of the third gill-cleft. Bom's i*esult has been confirmed bv C. Rabl, 86.1, Fischeles, 86.1, De Meuron, 86.1, F. P. Mall, 87.1, 88.2, Froriep, 86.1, 47, 91.2, 64, and Prenant, 91.2. His, 86.3, " Anat. menschl. Embryonen," III., at first maintained that the thymus arose from the ectoderm of the cervical sinus, but having made further observations finally reached the same conclusion as Bom, and showed, 89.2, that in man the thymus is derive<l from the third entodermal pouch. Kastschenko, 87.1, believed that the thymus was partly ectodermal, partly entodermal, an opinion which is incompatible with our present knowledge.

Thyroid Gland. — The thyroid gland is developed from three anlages, one me<lian and two lateral, which unite and undergo a common differentiation. We take up: 1, the median anlage; 2, the lateral anlages ; 3, their union ; 4, their differentiation ; 5, homologies.

1. The Median Aiilage. — This is an evagiiiation of the floor of tb« iihiiryiix between the hasen of the first and second branchial arubes; it liea in the median line behinii the tuberculum impar, p. 5'ri, Hini the fiircula, or the two parts of the tongue. In the human embryo, as v.o learn from His {"Anat. menschl. Embrj-onen," II., Ii4-Ta, i*T-HI'.i), tiie tivatpnation isasiuaU iwueh beginning to expand sideivavrt in im embi-yo of 6 mm. ; in an embryo of 10 mm. (c/. Fig, aas, hiJ/i) the lateral ex|>aiiijion bus increased verj- much and there is a distinrt, though narrow, median duet, the opening of which upon the surface o£ tbo tongue corresponds to the /orujxen C(^cm«j,- the duft itself is known as the ductus tlnjreiHjloa.iiis. The anlage now amsists i>f a biliiteral opithelial vesicle, connected by a slender, hollow i>etiicle with the surface of the tongne. The duct persists up to the eighth week, ^adually elongating as the thyroid and the tongue separate. The duct usually obliterates completely or partially, but it eoinetinies persists mom or less intact throughout life. The abortion of the duct begins usually during the fifth week, and when the anlage of tbo hyoid Ixme reaches the median line, it is situated directlj'in the i>ath of the duct, a topograph icjil rela
«i^r;,f'i"'iT,i!:;;,rsrriro'f"fl"r,^^Tii:?'i5";" tion of pathoiogirai impor
i-,^.?i;^-^/V:,^^^'fe;..^^^^^^^^^ tance {\\;^His, 91 1). He

3»>i ; «.«. mwlitiH Hiilaf.. of ilivnil.l: T^i. ili.vnm.^; abortion bi'gms With the CloK
S.;T"' ""■'■"" "»■"*'■ *"""•■'"■■ ^^ ure of tl,8 lumen of the duct; the solid cord gradually diminishes in size and Iwcomes fragmented as resorption progresses, but tbo up|)er jKirtion near the surface of the timgue retains its thickness for a time at least. Kanthack, in an article of slight value, 91.1, has denied without justification the existence of tho thyroid duct. The vesicular portion of the median anlage expands quite rapidly, Fig. i.iii, ms, and lies nearly at the level of the thin! aortic arch, 3, or internal carotid, and, indeed, is from the beginning in close proximity to tho larynx. In emhryie of i)-10 mm. it is a narrow, long transverse lv.n]y, th',' lateral ends of which curve dorsalward. and which, \\'itli the duct, form a figure si>mewhat like an inverted T,

The development in other mammals, so f;ir as known, is closely similar to that in man. Thus in the rabbit, Piersoi, 88.1, 182, found tho aidagc to ai)i)eai' the end of the ninth day (embryo of 2.:i mm,) ; the epithelium of the ll-.yroid evagination at once thickens and tbo anhigo bei-omes solid thet.i>iith day; the twelfth day the abortion of the duct begins ; and offer the sepjiratiou, not l)efore. as in man, the lateral outgrtiwth of tho anlage U-gius. In the pig, G. Bom, 83.1; inthe chick, Secsel. 78.1, and F. Mall, 87.1; in Amphibia, A. Gotte. 76.1; in Petromyzon, W. Miiller, 71.3, 78.1, and A. Dolirn. 86. 1, 87.2, have studied the median anlage of the thyroid, wliich may now \>e said to be n structure common to, and therefore

characteristic of, all vertebrates. The references just given might easily be multiplied.

G. Bom, 83.1, 301, found that in the pig the median anlage commences its histological differentiation and is penetrated by blood-vessels before it is joined by the lateral anlages. In man the differentiation is much less advanced when the union occurs.

2. The lateral anlages are derived from the epithelium (entoderm) of the fourth gill-clefts. The fourth entodermal pouch develops a ventral prolongation (human embryo of 10 mm., Fig. 435, Is). His (** Anat. menschl. Embryonen,^ III., 97) draws a distinction between the diverticulum and the pouch, but upon what grounds is not clear to me. In an embryo of 12.5 mm. (Nackenlange) His, /.c, 98, found the diverticulum a closed vesicle entirely separated from the phar}Tix; the vesicle curved forward and was just beginning to form a few round, hollow buds, and may now be designated as the lateral thyroid anlage. The median anlage at this stage is situated further toward the mouth and the ventral side. In an embryo of 13.8 mm. (Niickenlange) the lateral anlages have moved neaier the median, and take such a position that they prolong the median anlage forward and upward on each side.

In a pig embryo of 13 mm. Prenant, 91.2, 211, observed the ventral diverticulum of the fourth pouch still connected with the

pharynx, and records a similar condition for a bat embryo of mm. and a sheep embryo of 14 mm. Piersol, 88.1, 182, found the corresponding stage in a rabbit embryo of the eleventh day, and states that the anlage remains " for a relatively long time" connected with the pharynx by an epithelial cord. Born does not state clearly when the lateral anlages separate in the pig from the pharyngeal epithelium, but apparently the separation occurs in embryos of about 15 mm., compare Born, 83.1, 299.

3. Jjnio)i of the Three Anlages. — This was discovered by Bom, 83. 1 , 299. It takes place in the pig when the embrj^o is from 20 to 22 mm. long; the median anlage is at this time a network of epithelial cords and considerably larger than the lateral anlages, which have gradually clianged their position. Fig. 43G, A, B, C, D, until they have come to lie against the lateral ends of the median part, C ; with these ends the lateral parts then unite and soon acquire the same reticulate structure as the median portion, and there remains no evidence of the triple origin of the gland.

In man the union takes place probably during the seventh week — the exact time has not been recorded. The lateral anlages are relatively larger, and the median anlage less differentiated before the union in man than in the pig. As to the process of union in other mammals I find no precise data.

4. Differentiation, — In a pig embryo of 15 mm. (Born, 38.1, 301) the median thyroid is a transverse band of epithelium, around which the mesenchyma is beginning to form a capsule. The epithelial biind is beset with buds, which grow in such a way that the band soon lx3comes a network of epithelial cords. Fig. 430 ; the cords are solid with a superficial layer of distinct high cylinder cells with elongated nuclei, and surrounding a granular nucleated mass without distinct cell boundaries. At the same stage the lateral thyroid is merely an epithelial vesicle, at the ventral end of which the walls are thickened. After the fusion of the three parts one can still recognize (pig embryos of 20 mm.) the lateral portions, because, though now similar in structure to the middle portion, the epithelial cords , are thicker and the meshes between them smaller than in the middle part. In an embryo of 37 mm. (Born, I.e., 305) the gland has become an oval body inclosed in a smooth capsule of connective tissue.

His (•* Anat. menschl. Embryonen," III., 102) records that in a human embryo of the eighth week the formation of the hollow acini had begun; the acini were lined by epithelial cells with each an outer granular zone containing the imcleus, and an inner zone of clearer appearance. The outer zone stains more deeply than the inner. Wolfler, 71.1, has claimed that the hollow epithelial acini are formed by the degeneration of the central tissue of the solid cords, and in this conclusion he is supported by Lustig, 91.1, but whereas Wolfler maintained that the differentiation begins in the centre of the organ and progresses toward the periphery, Lustig asserts that the differentiation goes on throughout, so that mature and immature acini may be found in every part at once. Fig. 437 represents a section of the human foetal thyroid at about four months. It w^ill be noticed that many of the acini are still solid.

At two months the gland in man consists of two lobes connected by a narrow isthmus (Miiller, 71^3, 447). Miiller, /.c, also gives some details of the growth of the acini up to the period of puberty, as well as good observations on the foetal gland in various vertebrates.

0. Homologies. — The mammalian thyroid gland ia shown by its development to be a double organ. The median part is alone homologous with the Bo-called thyroid gland of other vertebrates, while

Historical Note. — That the thjToid gland arose from thepharjTix, and corament'ed as a thickening of the eiit«denn, was discovere^l by Remak, 50.1, &1 — 82. This diecovery was confirmed by Goette's observations on the chick, 67.1, and on Bonibinator, 76.1, 6(iT. W. Miiller's investigations, 71.3, 73. 1, added considerably to our knowledge of the median anlage in various classes, and led him to homologize the thyroid evagination with the hypobrauchial groove or endostyle of tunicates and Ann)hioxu8. This homology has found an earnest defender in Anton Dohm, 86.1. Seesel gave, 77.1. a more accurate description of the anlage in the chick, and it was also studied in man by His ("Anat. menschl, Emhryonen," I., 5fi), and in the rabbit by Kolliker ("Entwickelungsgeseh.," 2te Aufl., KTl). In 1881 L. Stieda, 81.1, discovered the lateral aniages, and trac:d them to a connection with one of the gill-clefts ; the same discoverjwas made the same year, but independently, by Wolfler, 81.1, who gives an extensive review of the previous literature. Stieda and Wolfler overlooked the median portion. Bom's thorough investigation, 83.1, finally cleared away the uncertainty by tracing out with rare precision the exact role of each part of the triple anlage. Born's results have since been abundantly confirmed by His, " Anat., Embryonen," III, 91.1, Von Kolliker ("Grundriss." 2te Aufl.. :!61l), Froriep, 86.1, De Meuron, 86.1, Piersol, 88.1, F. P. Mall. 87.1, 88.2, and A. Prenant, 91.2, 204-220.

The ceeophagus is developed from the short piece of the vorderdami, p. 211, between the phary;ix and the stomach. Fig. 441, oe. Duriny the fourth wt^k it begins to lengthen out, and by the end of the fifth week hiis become a cylindrical tube of considerable length, Fig. 444, C, As regards its further history we have little exact information. I have observed that during the fourth to sixth month it has usually four well-marked ridges formed by its mucous membrane, and that below the larynx these ridges are so arranged as to give the cavity of the (esophagus, as seen in cross sections, the outline of a Gi-eek cross, which

Balfour ("Works," III., 61) records that in shark embryos the cavity of the oesopha^s is entirely obliterated about the time the fourth gill-cIeft in formed, and so remains for a long period; the ob

literation is effected by the growth of the entodermal epithelium. That the entodermal canal is for a time in teleost embryos a solid cord has been already stated, and accordingly we find in them the cesophagus without a lumen during certain stages, cf. Mcintosh and Prince, 90.1, m. De Meuron, 86.2, states that the obliteration can be ob8er\'ed in anura, just after the larva hatches; in lizards, and the chick erabrj'o of five and onehalf days; in lizards the obliteration is incomplete. W. Opitz, 87.1, states that part of the lumen is closed in the human embryo, and concludes from that fact that the amniotic iiuid cannot be swallowed by the fcetus.

Stomach

The first trace of the stomach appears in a human embryo of five or six days as a slight dilatation of the entodermal canal. Fig. 441, st, between the cesophagus, oe, and the liver. /; the stomach at this stage is in the median plane and overlies the septum transversum. thedi- ^.^ latation continues to increase / ' nVSr during the whole fatal period. The stomach very early migrates into the abdominal cavity below the liver. Fig. 444, A, B, C, mere being a corresponding elongation of the cesophagus. In conse

iliieiice of this migration the stomach acquires a mesentery, ivhich on its dorsal side iw known us the mesogastrium, on the ventrai side aB the lesser omentum ; the mesogat^triiim is the anlage of the greater omentum or epiploon. During its migration the stomach also becomes iisymmetrical in shape anil position, Fig. 444, C; in that figure, which is taken from a tive weeks' embryo, the adult form of the stomach is clearly indicated; the figure also shows that the greater curvature belongs to the dorsal, the lesser curvature to the ventral side of the stomach. Finally during its migration the stomach also' revolves around its own axis so that its left surface is turned frontwai'd and its right surface backward, see Fig. 445, si, and moreover the ceplialic end of the stomach is on the left side, the caudal or pjloric end on the right side. In the change of position of the stomach we tind the explanation of the origin of the omentum by the folding of the mesogastrium, and also of the connection of the ventral mesenteiy or lesser omentum with the lesser curvature or primitive mediiui ventral Hue of the wtoniach on the one hand, and the liver on the other.

The revohition of the stomach around its own axis explains the asy linnet ricid position of the vagus in the adult, for the embryonic left side innervateii by the left vagus becomes the " anterior" surface, acconling to the descriptive anatomy of the adult.

HisTtMiBXBSIs. — (.)nr knowledge of the development of the gastric glands rests chiefly on the admirable memoir of Toldt, 80.1, who also reviews the scanty results of his pretlecessors. The entoderm of the stomach consists in young embryos (cat ;S0-130 mm.) of a cylinder epithelium, which gradually increases in thickness until the formation of the peptic glands begins (cat embrj-os of (il>-tu mm., human embryos tenth week). Groups of cells arrange themselves in miniature glands, which are contained entirely within the thickness of the epithelium; that is, they do not project into the mosodorm; each gland, Fig. 442, gl, when fulh- marked out, consists of ii small central cavity and a wall of finely granular cuboidal cells, and is separated from the neighboring glands by the unalteretl high cylinder-cells. This stage is descrilwd for a rabbit embrj-o of 4-2 mm. by E. SalvioH, 90.1. 7;i. The glands grow down into the mesiNli'nn (cat embrj-os of 85 mm.}, and one can sixin distinguish an upper portion or duct lined by high cylinder-cells and a lower glandular portion with a cuboidaJ epithelium, Fig. 443. The gland proper forms terminal and later lateral buds also, so that each duct acquires several branches, Fig. 443. The formation of new gland anlages ceases when the budding begins, but the glands continue to multiply, owing to the division of the ducts. At seven months the foetal stomach has about seven glands to each duct, and this proportion is kept until birth; but after birth, owing to the continued division of the ducts, the proportion is diminished ; thus Toldt found at ten years an average of six glands for each duct; at fifteen years five glands ; in the adult only three. The peptic cells {parietal or delomorphous cells, Belegzellen) arise by differentiation of single glandcells ; the differentiation begins by the accumulation of coarse granules (zj^mogen?), at first in the outer part, later through the whole cell ; these glandular cells first become recognizable about the time the branching of the glands commences (in man toward the end of the fourth month). The number of peptic cells increases both by division of the cells and the metamorphosis of the original cells. As the peptic cells are differentiated they take up their position on the outside of the gland. After the sixth month pepton may be obtained from the stomach. H. Sewall, 78.1, asserted that the peptic cells immigrated from the mesoderm, an error which, as Toldt has shown, was due to incomplete observation.

The niucotis glands (Toldt, 80.1, 119) appear about the same time (cat embryo of 50 mm.) as the peptic, as evaginations of the epithelium, which are lined throughout by cylinder — not by cuboidal — cells. Later the glands become branched. Kolliker ("Entwickelungsgesch.," 2te Aufl., 854) observed that the gastric glands measure during the fifth month from 0.13-0.22 mm. ; during the sixth 0.420.71 mm.

The spaces between the gland-openings become somewhat prominent during the third month, and these prominences have been described as villi by Kolliker and others, but there are no suflBcient grounds for maintaining that there are any true gastric villi at any period. The pseudo-villous appearance is most marked toward the pylorus, and persists at least through the fifth month. A little later than the villi there appear also on the inner surface of the stomach longitudinal ridges which vary in number from 12 to 15.

During the fourth month the inner circular muscle layer and the outer longitudinal layer become well marked (Kolliker, " Entwickelimgsgesch.," 2te Aufl., 853).

Intestine. — The intestine includes the whole of the entodermal canal from the stomach to the anus. Four entodermal organs are appended to it, the liver, pancreas, yolk-sac, and allantois or bladder. When first formed it is a short, straight, median tube, to the ventral side of which are apppended the yolk-sac and allantoic diverticulum, compare Chap. XII. The intestinal canal very early begins to elongate, and continues to do so throughout foetal life; while elongating it also increases gradually in diameter. I know no measures of the growth of the intestine. A consequence of its growth is that it has to form coils, which finally produce important anatomical changes. The posterior portion of the intestine increases more than the rest in diameter and becomes the large intestine (colon and rectum). From the anterior end of the colon jcrows out the ccecum, and from the coecum the appendix vermiformis.

General Growth. — The elongation and twisting of the intestine in the embr>'o ia indicated by Fig. 444. It has been carefully worked by W. His (" Anat. menBchl. Embryonen," III., 12-25). In an embryo of 4.2 mm. the stomach is barely indicated, A; the neck of the yolk-sac, i'ks, is very wide; nearly the whole space between the yolk-stalk and the stomach is occupied by the hepatic anlage, It. In an embr>-o of 7 mm., Fig. 444, B, the stomach has elongated and then to descend into the abdominal cavity; the yolk-atalk is con8i(ferably smaller, Vks; between it and the stomach the entodermal canal baa len^bened; near the stomach are appended the pancreas, P, and the liver, Li.d; the intestinal canal below the yolk-sac has also lengthened out, so that the intestine as a whole describes a long loop toward the veiitrfd side, to be there attached to the yolk-sac, see also Fig. 17; as thy stomach is situated entirely on the left side, it follows that the loop ia asymmetrical, the upper limb of the loop lying more to the right, while the lower limb lies more to the left. The ■ asymmetry ia more evident in later stages, Fig. 444, C, The upper limb, together with part of the lower limb, forms the small intestine; the division between large and small intestine does not coincide with the insertion of the yolk-atalk. The cephalic limit of the large intestine is first given in embryos of about 12 mm. by a small diverticulum, Fig. 445, B, Coe, the anlage of the coecum, compare also Fig. 444, C, Coe; the whole of the canal on the caudal side of the ccecum increases in diameter and forms the large intestine. Fig. 445, B, col. The small intestine now lengthens rapidly for a long period, and forma coils below the level of the coecum, Fig. 445, B; at the same time the large intestine, col, also lengthens, but more slowly, and its coecal end is carried over to the left side toward the cardiac end of the stomach, with the result that the small intestine has to cross ventrad of the lai^ intestine from right to left. The crossing of the two intestines introduces considerable com]tlexity into the arrangement of the mesentery, as explained in the next section. At the stage we have now reached. Fig. 445, the stomach, st, is relatively lai^, and has essentially its adult form, but Can . it still lies almost wholly on the \'^'^\ w^st™ left side; its pyloric end is to the ^^^J^^"' right of the median line; from the pylorus springs the duodenum or beginning of the small intestine; it runs toward the median line nearly parallel with the greater curvature of the stomach ; the liver duct, Z/, and pancreas, P«/(, are both connected with the upper end of the duodenum ; the pancreas liea, as Btatod, p. T(I7, in a transverse position between the duodeinimand thestomach. The small intestine makes several coils and terminates on the right side of the Ixxly by joining the colon; in Fig. 445, B, however, the end of the colon lies on the left, but this is unusual and was, ix?rhaps. a case of partial reversus situs viscerum.

As the largp inietifine grows, its coecal end descends toward the pelvis on the right side, and it may then Ix; sulxlivided into the four parts recognized in descriptive anatomy, to wit: 1, 2, 3, the ascending, transverse, and descending colons ; and 4, the rectum.

Fto. . - Two Front Views of

Caecum and Appendix Vermiformis. — The caecum arises as an outgrowth of the ileal end of the large intestine ; it appears in human embryos of 10-12 mm. The appendix appears as a blind outgrowth. Fig. 440, T", of the coecum. At six months. Fig. 446, it is long and slender, with a narrow free mesentery and is relatively much better developed than in the adult, and also is less sharply marked off Yrom the ccecum projier.

Intestinal Hernia. — By this term we may designate the normal temporary extrusion of the intestinal canal into the umbilical cord. So far as I can now recall this extrusion has been observed only in man. In human embryos of 10 mm. the part of the intestine attached to the yolk-stalk begins to enter the umbilical cord, and thereafter the length of the intestine, which leaves the body cavity proper and lodges in the coelom of the yolk-stalk, increases until, perhaps, the tenth week. thereafter it is gradually withdrawn into the abdomen. The cause of this temporary umbilical hernia is believed to be the strain produced by the yolk-sac ; attention is directed to it in the descriptions and figures of embryos in Chapter XVIII.

Histogenesis. — The intestinal canal consists at the end of the first month of an inner layer of entoderm and an outer layer of mesoderm ; small ccecum ;"/. inr large the former bccomes the epithelium of the villi intestine. Natural size.

and glands, the latter gives rise to the connective tissue of the villi, mucosa, submucosa,.etc., and also to the two muscular layers and to the peritoneal covering. The epithelium is a high cylinder epithelium like that throughout the undifferentiated entodermal canal. The mesoderm is a thick layer of mesenchyma covered externally by the cuboidal epithelium (mesothelium), which lines the coelom, except that the anal end of the intestine (future rectum) has no mesothelium because it lies beyond the coelom.

At two months I find the villi and glands of the small intestine beginning their development. Fig. 447, and all the layers of the mesoderm sufficiently differentiated to be recognized. The stratification of the intestinal mesoderm can be recognized in a cat embryo of 25 mm. according to Patzelt, 83.1, 14C. The villi, 17, are short, thick, and few in number, but additional villi are developing between those already present; the entoderm has altered its primitive character very slightly ; the epithelial glands are to grow out between the bases of the villi. The villi also appear throughout the large intestine, but are obliterated (Kolliker "Grundriss," 2ie Aufl., 360) there by the upward growth of the glands, while in the small intestine the villi enlarge and persist throughout life. C. von Langer, 87.1, 54-50, studied the mesodermal cores of the villi and found them well developed in the large intestine during the fourth month, partially aborted at birth, and completely aborted one month after birth. The mesothelium, msih, has begun to thin out to eonvert itself into the peritoneal epithelium, but the connective-tissue layer of the peritoneum is not yet recognizable. The two muscular layers, Im, cm, are marked out by the elongation of the mesenchymal cells to form smooth musclefibres. The submucosa, conn, consists of dense undifferentiated mesenchyma; its thickness about equals that of the entoderm,

Fig. 446. — Part of the Intestine of a Human Embryo of about six Months (Mi not Coll., No. 65). 8 in. Small intestine; \\ vermiform appendix, with its free mesentery and arising from the

En I, or that of the two

muscular layera, Im, ;|5Sfi,',-.|aa::l<j; cm, taken together.

The eutodenn often contains leucocytes, it gradually loses its embryonic character.

Human Eiiitiryo . No. iW.) mit.

-, IretiKnuPof oilf
uRcles: rn>. clrrulu- uiusclM ; I'i,

After the second month Over the villi of the small intestine. Fig. 448, it becomes a beautiful cylinder epithelium with basally placed nuclei, which all lie nearly at one level, in marked contrast to their earlier distribution. The villi themselves are more or less cylindrical in form with the free ends rounded. In the sections, which I have examined, the entotlermal villi are only partially filled with mesoderm^ — a peculiarity which I am inclined to regard as normal, not as artificial. The glands begin to arise early in the third month (in the rabbit, when the embryo is about 45 mm., Barth, 68.1, 131). They are hollow outgrowths of the entoderm (Barth, Ix.; Patzelt, 83.1), extending into the mesoderm ; for a qonsiderable period they remain short as compared with the villi, see Fig. 448. The development of the glands of the small intestine has been imperfectly studied; Barth, 68. 1, 133, states that the glands of Brunner may be recognized by their branching in rabbit embryos of 70 mm. The glands of the large intestine have been studied by Patzelt, 83.1, principally in the pig and rabbit embryos, which he found more favorable than human embryos; the entoderm in cat embryos of 33 mm. contains small groups of short granular cells, with oval nuclei with nucleoli ; these groups are gland anlages, and are easily recognized by their pale nuclei ; the anlages are separated from one another by lines of cells with longer nuclei, which stain more darkly with h^ematoxylin ; but in embryos of 50 mm. and older all the nuclei stain nearly alike. The villi of the large intestine are temporary; they have been shown by C. von Langer, 87. 1, o4-5G, to be united by ridges running between their bases ; the ridges subdivide the surface into little areas, and in each area lie several glands; in the human foetus the ridges are still present at term, biit disappear in the course of the first month after birth. the gland anlages grow slowly — in the cat at birth they are only 0.23 mm. Both the anlages and the young glands multiply by division, which begins at the lower end of the gland and spreads to its mouth. Patzelt found in a section of the large intestine of cat embryos of

33 mm., 6-8 glands. 50 '• 14-17 GO " lG-10 G8 " 21--^3 82 " 40-42

1)5 mm., 45-50 glands. 101 " 54-57 114 " G7-70 140 " 110-120 "

The first beaker-cells of the large intestine appear on the villi (cat embryos of GO mm.), they rapidly increase in number, so that in the cat most of the entoderm consists of beaker-cells both over the villi and in the glands.

Growth of the Intestinal Entoderm. — The gland anlages and later the fundi of the glands are the centres of growth for the intestinal epithelium, as first suggested by Pfitzner's observation that the karyokinetic figures occur chiefly in the glands, not generally over the epithelium (Arch. f. niikrosk. Anat., XX., 137), but the definite recognition of the fact is due to Patzelt, 83.1, 1G5. The multiplication of cells in the glands of the intestine and stomach is confined in the adult to the fundi of the glands. That the bottom of each gland is a separate centre of growth was, I think, first suggested by W. Flemming, 86.2, and has since been fully demonstrated by the researches of Bizzozero and Vasale, 85.1, Heidenhain, 88.1, 2G-28, Bizzozero, 88. 1, 89.2, and E. Salvioli, 90. 1. I consider that the notion of discrete centres of growth in epithelia, with its corollary of translation of the cells from their place of origin, is an important advance in our conceptions. It is probable that other glands also ^row in the embr^^o as in the adult, but no direct observations on this i)oint have yet been made.

The Liver, — The early development of the liver has beeu described p. 21)8, and its situation in the septum transveraum explained. O. Hertwig ("Lehrb. d. Ehitwickelungsgeach.," 3teAufl.) describes the liver aa being primitively lodged in the ventral mesentery — an error of statement for which I cannot account.

The liver of all vertebrates consists of tv^o parts: 1, a branching system of epithelial gall-ducts, and 2, a network of hepatic cylinders. The two parts are morphologically distinct. The gall-ducts are surrounded by connective tissue, and, as is well known, are accompanied by the branches of the portal vein and hepatic artery. The hepatic cylinders are separated from one another only by endothelial blood-vessels. The essential primitive features of the hepatic cylinders are illustrated by Fig. 441'; every cylinder, hp. is an epithelial tube with a small central lumen and covered by an endothelium, which is easily recognized by its flattened, darkly stained nuclei ; the endothelium is the wall of a blood-vessel or channel, hi. The hepatic cylinders by branching and uniting form Tip-w_. a network, all the meshes of which are entirely occupied by blood-vessels. In sharks, Fig. 449, each cylinder comprises in its cross section usually eight to ten cells, and is almost completely bathed in blood. In amphibia the cylinders are smaller; they compriseonly four to five cells in cross section, and their lumen is very small, and the ofan AcanthiaaEnibrroaiasinm. a/>, h^blood-channels between them are relatively diminished. In mammals each hepatic cylinder comprises merely two epithelial cells; the lumen is reduced to a minute canal (the gall capillarj-) ; the cylinders anastomose with one another very frequently and at very short intervals ; and Anally the blood-vessels between the cylinders become smaller tor the most part than the cylinders. In mammals we have further the hepatic cylinders gathered into radiating groups; the groups are the lobules of descriptivoanatomy. In most text-books the mammalian hepatic cylinders are referred to as " radiating rows of liver cells." If the fundamental notions just recapitulated are kept in mind the following paragraphs can be better understood.

Fig. 449,— Portion of tt SMiion of the Llver

The liver commences, as stated p. 2(18, as a diverticulum of the entoilermal canal extending into the septum transversum. This single median diverticulum may be designated as the Aniphioxus stage, since a similar diverticulum in the cyclostome is reganled, probably correctly, as the homoiogue of the primitive hepatic aulage of true vertebrates. The single diverticulum develops to a considerable size in shark and amphibian embryos, but in amniota it forms two branches almt^t immediately (chick fifty-five to sixty hours, rabbit eleventh day), so that it is usual ti> deftcril>e the amniote liver as arising from two diverticula. The evaginations are, of course, lined by entoderm; they are situated in) mediately behind the heart, and embrace between them the two vitelline veins forming the roots of the ductus venosus. In the chick the right pouch is from the first longer, but of smaller diameter than the left (Foster and Balfour, " Elements," 2d ed., 179). In the rabbit, according to KoUiker ("Grundriss," 2te Aufl., 372), the left pouch appears the tenth, the right the eleventh day. In the human embryo of 3 mm. His, 81.1, found the hepatic diverticulum single.

In the primitive form of vertebrate development (Petromyzon and amphibians) the hepatic diverticulum extends into a mass of entodermal yolk-cells, so that it has from the start several layers of entodermal cells around its cavitv. The cells form a mass which, as described by W. T. Shore, 9l'.l, i:0-18:3, separate off (in the frog, at least) from the rest of the yolk, the cells themselves multiplying and changing into liver-cells. They constitute a thick, solid wall around the hepatic diverticulum; channels appear in the solid walls, and these channels acquire endothelial linings, and blood enters them ; the yolk-cells between the blood spaces gi'adually develop into hepatic cylinders. These changes can l)e favorably studied in a frog's tadiK)lo six or seven days after hatching.

In amniota there is an early separation of the liver anlage and yolk-sac, and the former has thin walls when it arises. W. T. Shore, 9 1 . 1 , 1 84, states that in the chick the walls of the diverticulum begin to thicken ahnost immediately by the proliferation of the cells, and in the thickened mass channels appear," there take place irruptions, as it were, of capillary blood-vessels from the vitelline vein into the solid mass of proliferated hypoblast (/.e., entoderm), breaking it up into more or less branched rods of cells (second half of the third day)." In most text-l)ooks the hepatic entoderm is described as sending out solid buds between which the blood-vessels arise, but it is doubtful whether such a description is accurate. I strongly incline to accept Shore's view that the solid anlage is broken up by the formation of blood-vessels in it. If Shore is right we can imderstand why the hepatic cylinders form a network. So far as known the hepatic cylinders are at first solid and do not acquire their lumen until later. In the later stages of incubation the liver has the color of the yolk. In a chicken just hatched the liver-cells contain oil drops.

In mammals the development of the liver is similar to that in the chick. The walls of the primitive diverticulum thicken, become j)enneated by blood-vessels, and so divide into hepatic cylinders. Fig. ioo. The cylinders are at first solid and quite irregular in shape and size. Fig. 450, hp, and the blood-channels, 6/, are verj' large. The differentiation of the cvlinders in the Imman embrv'O has been studied by Toldt and Zuckerkandl, 76. 1. They found the cylinders to have a lumen in a four weeks' embryo; from this age till the end of fcx^tal life the c^'linders contain two forms of cells: 1, large polyhedral cells, resembling those of the adult organ; 2, smaller round cells, the nuclei of which stain darkly ; the two forms are mingled irregularly; the smalkr cells entirely disappear after birth and are )>resumably only a young stage of the liver-(*ell. It is not until some time after birth that the cylinders assume the adult mammalian type ; they become longer and slenderer, not, however, by s change in the size of the liver-cells, but by a rearrangement of the cells, such that the number of cells in a cross section of a cylinder is gradually reduced to two; the cj'linders after this change are zig-zag, but Boon etraigbt«n out. The metamorphosis takes place irregularly, so that several st^es can be seen under the microscope in one field of view. As regards the development of gall-ducts, we have no definite knowledge. We may surmise that they arise as evaginations of the primitive diverticulum and are always distinct from the hepatic cylinders.

Fig. 450,— Soctlni

Lobules. — Toldt and Zuckerkandl, 76.1, have investigated the changes in the blood-vessels in the human liver. In a four weeks' embryo the vessels are sill large, compare Fig. iSO, but by the eighth or ninth week the main efferent and afferent stems are recc^piizahle. For the history of the metamorphosis of the lar^ veins passing through the liver see p. 5-15. During the third or fourth mi)nth the vascular territ^tries of the portal and hepatic veins become distinguishable, for the branches of the two veins distribute themselves so as always to be separated. There now appear, scattered through the liver, islands (^f tissue with abundant fine ramifications of the hepatic vein; each island is the aniage of a group of lobules, and is surrounded by portions of the liver containing the branches of the portal vein. The portal system cuts into the island, so as to divide it gradually, while it expands, into iobnlea, and these primary lobules are similarly subdivided until the permanent lobules are established. The lobules enlarge after the production of new lobules has ceased.

Growth. — The liver enlarges very rapidly, compare Figs. 1711, 223, 4-14, and 451, and consequently has to project from the septum transversum into the abdominal cavity more and more. It forms two lobes, one each side and connected across tbeme<]ian line; between the two lobes, Fig. 451, r.H and I.Li, is situated the

Fig. 451. — Section of a rabbit embryo of thirteen days through the region of the fore limbs and liver.

. Wrf, Si.innl mnl; great vein, ]>, <if the liver; as that vein is constituted partly by the inubilical vein, it ia attached to the ventral body-wall of the embi'vo. In a rabbit of thirteen days lx)th !ol>es are well ilevelojwd and project l)eyond the level of the umbilical vein, but in the mithan line the liver is entirely on the cephalic side of the vein. In longitinlinal median sections this shows very clearly, as does also the fact that the liver is an appendix of the septum transversum. While the liver is exi)anding the utomach migrates into the abdominal cavity; after that migration we find the stomach connected by a thin membrane, or ventral mesentery, with the median dorsal line of the liver; the membrane extends forward to the septum transversum and joins it; the membrane is the anlage of the omentum minus; concerning its development we possess no accurate information beyond the fact that it arises after the first differentiation of the liver and stomach, and is a new structure produced as the stomach and liver descend into the abdominal cavity. Similarly we find on the ventral side of the liver there is developed a mesenterial membrane by which the liver is bound to the median ventral line of the somatopleure ; this membrane is the anlage of the suspensory ligament; posteriorly it extends at least to the imibilical vein, anteriorly to the septum transversum, with which it is continuous. The liver now has the follow^ing attachments : 1, by the omentum minus to the lesser curvature of the stomach ; 2, by the suspensory' ligament to the median line of the body iind the inferior surface of the ventral part of the septum transversum (or future diaphragm) ; 3, to the dorsal part of the septum transversum. The connection with the septum transversum is both j)rimitive and permanent, so that in the adult the liver may 1x3 described as an appendage to the diaphragm. But whereas in early embryonic stages the attachment of the liver occupies nearly the entire septum, in later stages the septum develops over a considerable expanse, so that the attachment becomes relatively smaller. Fig. 455, and is confined to the dorsal region of the septum or diaphragm. The area of attachment finally becomes round, with two lateral prolongations; the round part is the coronary ligament^ while the prolongations are the lateral ligaments of descriptive anatomy.

As the liver grows in the septum it is, of course, covered by mesothelium,and as it enlarges and becomes a more in(lej)endent i)rojection it retains its mesothelial envelope. Fig. 450, nisih. Later a layer of mesenchyma is developed between the liver-cells and the mesothelium, and the two mesodermic layers together constitute the peritoneum , As to the histogenesis of the hepatic peritoneum we have no accurate information. From the mode of development of the liver it is evident that ^'r,?^ the mesothelium, and /a r<er the peritoneum, covering the liver must be directly continued on to the ligaments of the liver, the diaphragm, and the lesser omentum.

For illustrations of the growth and position of the human foetal liver see Figs. 153, 170, 250, 284, 303, 305, 310. During the second month it becomes of relatively enormous size; so that during the third month it extends far into the hypogastric region and fills out the greater i)art of the abdominal cavity. After the fifth month the intestines and other viscera overtake the liver, but at birth the liver makes two thirty-sixths of the total weight, as against one thirtysixth in the adult. Immediatelv after birth the liver diminishes in size (Kolliker, Entwickelungsges.," 2te Aufl., 880). The right lobe of the liver is probably always larger than the left; after birth its predominance increases.

Another important factor of the development of the liver is the atroph}' of the hepatic cylinders in certain j)arts, as discovered by Toldt and Zuckerkandl, 76.1. They have observed this atrophy near the lateral and suspensory ligaments, next the gall-bladder and in the neighborhood of the umbilical vein. When the atrophy begins the liver-cells become finely granular, opaque, and lose their outline ; the protoplasm breaks down and disappears ; the nuclei persist a little longer. Changes occur also in the gall-ducts of the atrophying regions.

Functions. — I cannot do more than allude to the manifold and important functions of the foetal liver. For its sanguinif active role, see Chapter X. For a general discussion of its physiology, see W. Preyer, '* Specielle Physiologie des Embrj^o." For speculations up<:>n the relation of its functions to its mode of development, see W. T. Shore, 91.1, who also makes suggestive remarks as to the evolution of the liver. In regard to the glycogenic function of the liver in the embryo, see especially Claude Bernard (C. R. Acad. Sci. Paris, XLVIIL, 77-86).

The gall-bladder arises in the chick during the fifth day as an evagination of the right primary diverticulum. KoUiker observed it in the human embryo during the second month, and saw folds on its inner surface during the fifth month.

Pancreas. — In amphibia, there are three pancreatic evaginations ; one dorsal, and two symmetrically placed on the ventral side close to the ductus choledochus ; the triple anlages were first discovered by A. Goette, 75.1, in Bombinator, and have since been studied in Triton, Siredon, Rana, and Bufo, byE. Goppert, 91.1, 113-118.

In the chick the pancreas, as described by Foster and Balfour (" Elements," 2d ed., 181), arises during the fourth day," in the form of an almost solid outgrowth from the dorsal side of the intestine, nearly opposite, but slightly behind the hepatic outgrowths. Its blind end becomes somewhat enlarged, and from it numerous diverticula grow out into the passive splanchnic mesoblast. As the ductules grow longer and become branched, vascular processes grow in between them, and the whole forms a compact glandular body in the mesentery on the dorsal side of the alimentary tract. The primitive outgrowth elongates and assiunes the character of a duct. On the sixth day a new similar outgrowth from the duodenum takes place between the primary diverticulum and the stomach. This, which ultimately coalesces with its predecessor, gives rise to the second duct, and fonns a considerable part of the adult pancreas. A third duct is formed at a much later period.

In mammals only the single pancreatic evagination was known until recently. Its development in man is thus described bj'O. Hertwig ("Entwickelungsgesch.," 3to Aufl., 280) : The dorsal anlage appears a little later than the hepatic diverticulum ; it has been observe<i by W. His in embryos of 8 mm. as a small diverticulum, Fig. 44-4, B, P, which grows into the dorsal mesentery, sending out meanwhile hollow, branching buds. Fig. 444, C, P, and thus becoming by the sixth week an elongated gland, which extends so as to lie in the mesogastriiim or future omentum, and, therefore, between the greater curvature of the stomach and the vertebral column. The pancreas, therefore, changes its position as the omentum develops; thus at six weeks it lies parallel with the longitudinal axis of the body; thereafter it revolves so that its anterior end moves to the left, as the omentum develops, until the gland occupies its permanent transverse position, and the so-called head of the gland lies in the bend of the duodenum, while the so-called tail is near the spleen and the left kidney. The duct of the pancreas is at first in front of the bile duct, but during fcetal life it shifts and first approaches and then joins the ductus choledochus.

Stoss in a preliminary notice, 81.1, states that in mamnials he has found the dorsal and double ventral fiancreatic anlages (sheep embryos of 4 mm. about seventeea to eighteen days) . _ The two anlages unite (sheep of 15 mm.) ; the duct of the ventral aniage is the ductus Wirsingiamis, of the dorsal aniage the ductus Santor I'vi. In sheep and man the ventral duct is preserved; in the horse and dog both ducts ; in cattle and the pig probably the dorsal duct only. In sheep the lumen of the dorsal duct is obliterated in embryos of TO mm., and in embryos of i'O mm. only the ventral duct can be found.

As regards the relations of the pancreas to the ijeritoneum: the entodermal portion of the pancreas being situatetl in the mesogastrium, it is, of course, covered on both sides by peritoneum and may be said to be attached to the wall of the abdomen by a mesenterj' of its own, although the pancreatic mesenter;- is only a part of the mesogastrium (C. Toldt, 89.1). The pancreatic mesentery aborts during the fifth month, and the pancreas, losing its movabilitj', becomes directly attached to the dorsal abdominal wall.

The histogenesis of the pancreas is still to be investigated. In a human embryo of four months, Fig. 452, the alveoli show clearly and lie in groups — drawn dark in the figure — which are widely separated from one another by young connective tissue. The ducts are lined by a cuboidal epithelium; the cells of the alveoli are small, containing very little protoplasm, but each having a well -developed spherical granular nucleus.

Mesentery and Omentum. — To understand the development ot the mesentery it is necessary to recall the fact that the ventral portion of the coelom, or, in other words, the splanchuoccele, is constituted by a pair of cavities (pleuro- peritoneal spaces), which are . separated from one another throughout the Ixxly by a median partition or mesentery. Fig. 453, A, iite.t: in this pai-titiou is lodge<l the entodermal canal, entj the partition consists nf mesencbyma and is, ot course, covered on both sides by mesothelium, tiisth. The con

nectioii (if tlit* imsentor} with the Buinatopleure along the median ventral line is Uwt for the moat part verj' early, but tiie stomach is always ciniiiecte<l by a ventral mesentery (omcDtiim minus) with the ventral botly wall. The partial disappearance of the ventral mesentery establishes the c< ndition indicated 1 j F g, 453, B; the entodermil tulje, together with the mesoderm around it, constitutes tl e alimentarj' ctinal, which is su8iK,iided by a dorsal permanent mesentery from the median Ime til 'coelom, ci>e, of one side c n municiit^s I)elow the intestine M ith the coelom of the opposite side. In other wonls, by the disappearance of the ventral mesenterj- the paired splauchnocoeles have fu8e<i, and there is hei cefortli a single abdominal cav itj

In the cephalic r^ion of the abd men, however, the primitive c mplete separation of the CLelom f the two sides persists. As tl e stomach and liver descen 1 from the septum transverloncwd' sum or primitive diaphragm. _. ™, /*odepn* *^ Iward into the abdominal

?' cri'm" hIm "oni- unmnus ma /i n fw cavity we find that the meseothelium. L: 1i^,r .1. s,»,«.n«.ry liRai^rnt. ^^^^^ mrtitioU gTOWS with them and is never alx>rted either on the ventral or dorsal side. Four orgaiiH are liwlged in this partition, Fig. 454, the spleen, Spl, pancreas,

Besides these names there are also emplojed viesogastriHm for the embryonic greater omentum, and vic.iocoloii for the portion of the mesentery connected with the large intestine.

the condition just described is readied by the human embryo during the fourth week. Concerning the mode of disappearance of the ventral mesentery I can recall no exact observations, nor do I know of any satisfactory descriptions of the earlj- stages of the partition in which the stomach, etc., aro lodged. We are, therefore, forceil to content om-seives for the present with the preceding diagrammatic explanation. The diagram, Fig. 455, will serve to render both the preceding account and the siilieequent changes clearer. TJie diagram is fairly correct, except in representing the stomach in the median lino, for as soon as the stomach descends it takes an asymmetrical position, p. 754. It will be evident upon glancing at the diagram that the mesogastrium, may, mesentery, msl, and mesocolon, msc, are merely different regions of the same membrane, and that the spleen, spl, pancreas, '^^•^ pan, stomach, ni, and liver, Li, nch; Wg. meBoicvi are l<x;ated in one complete mesen- "'»'■ ""■*^'"*-'>' i """"■ terial partition, so that in the region of the stomach and the liver, in order to get from the left splanchnocceie to the right splanchnocoele, we must pass around the liver on the caudal side ; the septum transversum, st, prevents our passing across on the cephalic side.

The further changes in the relations of the mesentery depend chiefly on two factors : first, the elongation and coiling of the stomach and intestines; second, the formation of secondary adhesions ot certain parts of the mesentery with other parts and with the abdominal wall.

pp. t53, 755. The result of the primary twisting upon the mesentery ia illustrated hy Fig. 450. Owing to the deflection of the stomach to the left, and of its revolution around it« axis, by which its median dorsal line or greater cun-atiire lieeoraes lateral, the mesugastriimi, msg, ia folded so as to form a |)oiich that projects toward the left Bide ; the iwuch is the anlage of the great omentum, Om : it oiM'ns toward the right side, its t>pening being the foramen of Winslow ; the inner surface of the pouch is formed by the riglit surface of the mesogastriuni, the outer bv the left. The cavity of the lM)uch may be termed the umenial cavity {Xetzbentel); F. P. Mall, 91.2, terms it the gastric diverticulum; in descriptive anatomy it is known as the les.ser perHoiieul njxice. From the lesser curvature of the st*imach extends the ventral mesenterj- or leaser omentum; an inspection of the diagram. Fig. 45ii, will show that it extends the pouch of the omentum toward the right. A section of a human embryo in which the timental cavity is just l>eginning to form is figured by F, Mall, 81.3, -(74. The diiodouum is situated near the dorsal aide of the body cavity and has, even in the young embrj-o, only a short mesentery ; as development progresses the duodenum, after making its pyloric bend, comes to lie in IX nearly transverse direction close to the dorsal abdominal wall ; its mesentery obliterates, and thereafter the duodenum forms merely a slight projection covered by mesot helium (and later by jwritoneum), compai-e Fig. 4.'>7, A, Finally, owing to the intestines forming a great l<Kip to the right, the large intestine crosses the body on the ventral side of the duodenum; the mesentery- meanwhile remains attached along the meilian dorsal line, but its ventral border elongates with the intestine; and further, the manner in which the k>op is devekiped brings the right surface of the mesenterj- to face ventralward (or "forwanl," according to human descriptive anatomy) and the loft surface to face dorwalward.

The additional changes are indicated by the two diagrams after O. Hertwig, Fig. 4.iT. A, B. The star (•) is placetl in the omental cavity. In A the liver, /, is attached to the dorsal part of the diaphragm, zf; tho stomach, mtf, occiijiies a transverse position, and is, therefore, .-M-en in cross sections; along what wjis primitively its median ventral line is attaehetl the lesser omentum, kn, by which the stomach is connected with the liver. Along the greater curvatiin*. !/(', of the stomach is inserted tho mesogastrium, or, as we may now call it, the greater omentum; it has grown so much that it forms a fold, which is heginning to hang over the transverse colon, ct; the fold ia destined to grow still further, as indicated by the dotted line, gn'; the pancreas, p, lies in the omentum close to the dorsal wall of the abdomen. The duodenum, du, is already closely united with the dorsal wall. The transverse mesocolon, msc, springs from the wall of the abdomen between the pancreas, p, and duodenum, dit; the reason for this apparent anomaly will be understood by referring to Fig. 45G. Below the duodenum, d%i, springs the mesentery', mes, of the small intestine, dd. In B the omental fold has extended, gti', far down in front; the mesocolon, msc, has united with the part of the omental fold nearest it, and there results a single membrane of double origin, by which the colon is suspended; it is this membrane which is known in the descriptive anatomy of the adult as the mesocolon; the adult mesocolon, therefore, includes the true mesocolon and part of the mesogastrinm. As a further result of the secondary adhesion, wo note that the omentum, gn*, appears to spring from the transverse colon, ct. Both the pancreas, p, and duodenum, rfu, now occupy their permanent {or so-calleil retro-peritoneal) positions.

Admirable descriptions of the condition of the mesentery at successive stages in the human embryo are given by C. Toldt in his classic memoir, 79.1, and further valuable observations on the adhesions are recorded by Toldt in his second article, 89. 1 .

Histogenesis. — From its nnHle of formation it is evident that the mesentery is primitively a sheet of mesencliyma covered on both sides by mesothelinm. the differentiation of this simple membrane has been carefully traced by C. Toldt, 79. 1. 43-50, At four weeks the mesenchymal cells are very much orowded^ — there being but little basal substance — and they have but little protoplasm ; some of them are beginning to assume the spindle shape ; the mesothelium varies somewhat, being here a cut>oidal, there ii cylinder epithelium. At six weeks more of the mesenchymal cells are spindle-shaped, and the mesothelial cells are beginning to flatten out; they are thinner and wider and their nuclei protrude. At eight weeks the mesothelium has essentially the endothelial tyjx?, which it retains thoughout life. At eight weeks the mesenchymal cells next the mesothelium on each side commence to form a special recognizable layer, which is perfectly distinct by the end of the third month; this layer is four to six cellsr thick and contains no vessels; together with the overlying mesothelium it constitutes the peritoneal membrane of descriptive anatomy. Between the two peritoneal membranes lies the looser mesenchyma, corresponding to the membrana propria mesenterii of Toldt, and in which are distributed the blood-vessels and nerves, and later (fifth month) the 13'mphatic glands and fat-cells. The mesentery thus comprises five layers, all of which can be well seen in embryos of the fourth month. The development of the connectivetissue fibrils in the omentum has been previously descrilxnl, p. 400. The fat-cells do not attain their typical development until the end of the eighth month, though their differentiation begins during the fifth, when the anlages of the lymph-glands also appear.

The mesodermic layer of the peritoneum is always very thin, but Toldt, 79.1, 4G, distinguishes in it toward the end of foetal life three sub-layers, viz. : 1, next the mesothelium with fine elastic net- work; 2, middle sub-layers with coarser elastic network; 3, subserous layer of lcx)ser texture uniting the peritoneum to the niembrana })ropria.

Meshes of the Omentum. — After birth the omentum becomes pierced with numerous holes. A few months after birth (C. Toldt, 9.1, 41)) there can be seen numerous scatteretl 8ix)ts where the membrane is thinner and contains fewer connective-tissue fibrils than elsewhere ; these spots lie more or less remote from the bloodvessels. At these six>ts the holes are formed and are at first always very small. Tlio formation of the omental perforations may l)e followed in children of from a few weeks to four years old. If the omentum of a child a few months old is stained with nitrate of silver there will appear, between the mesothelial cells, sjxjts colored by the silver; then other spots similarly coloreil, but larger and light in the centre; and finally still larger ones in which the light centre hiis become a hole, Toldt, /.r., Fig., IT. Toldt regards the holes as the result of the distention of the membrane, and the silver marks just described as indicating the pulling apart of the endothelial cells : the bl(K>d-vessels and fat-cells around them serve to maintain the thickness of the membrane between the holes. Ranvier, 74. 1, sought to attribute the origin of the holes to leucocytes forcing their way between the omental tissues, but Toldt has shown that tliis explanation does not hold good.

Historical Note. — The foundations of our knowledge of the embryonic mesentery were laid by J. F. Meckel, 17.1, and Johannes Miiller, 30.1. But little was added until the investigations of C. Toldt, whose two memoirs, 79.2, 79.1, constitute the classic authority on the subject. Lockwood's article, 84. 1, did not add much, and contains important errors, as ix)inted out by Toldt, 89.1.

II. The Respiratory Tract

The respiratory organs arise as a single evagination of the entodermal canal on the ventral side of the caudal end of the pharynx. The evagination branches, each branch develops into a lung; the main stem becomes the trachea, and the opening of the stem into the pharynx forms the larynx. Accordingly we take up: 1, the pulmonary anlage; 2, the lungs; 3, the trachea; 4, the larynx and epiglottis.

Pulmonary Anlage. — The first trace of the outgrowth of the entoderm al canal to form the lungs is an increase of the vertical diameter of the canal in the oesophageal region. This increiise results from the development of what may be called the pulmonary groove^ which is a furrow lined with entoderm, and begins just behind the fourth gill-cleft and extends to the stomach. Fig. 458. The groove is shallow toward the pharynx and deepens toward the stomach, ending abruptly as a rounded projection. The entoderm lining the pulmonary groove is thicker than that lining the cBsophagus above it (W- His, 87.3, 90). This stage may be seen in a human embrj^o of 3.2 mm., or in chick of sixty to seventy-two hours. The pulmonary groove is narrower than the oesophageal division from which it springs, hence in a cross section of this stage the entodermal canal has in the region of the oesophagus the outline of an inverted pear. The groove now deej^ns and its gastric end g^ows out farther, Fig. 458, Lti, and the oesophagus between the end of the groove and the stomach begins to lengthen. Presently the blind, free, lower end of the groove widens out, and, as it grows, forks; each fork is the anlage of a lung, and has the form, Fig. 438, of a short rounded pouch situated laterally. In front views the tw'o pouches are easily found ; in a side view one hides the other. The median portion, by which the pouches are connected with the

pulmonary groove, is the anlage of the trachea. As is well shown in Fig. 444, A, B, C, the cesophagus, lungs, and trachea all grow rapidly; the branching of the lungs begins in embryos of seven millimetres, B. In embryos of seven millimetres the fundamental parts are all marked out, except that the dilatation of the upper end of the trachea follows later (embryos of 13 mm. Fig. 444, C, La). The elongated oj^ening of the pulmonary' gr(X)ve is the future glottis, and in front of it is the anlage of the ej)iglottis, Fig. 444, C, Ep; the meflian cylindrical tul>e, Fig. 445, Tr, is the trachea, and its two branches are the lungs.

The situation and topogra|)hical relations of the pulmonary anlage are very importimt, because they explain numerous anatomical facts.

At first the anlage directl}' overlies the heart and the septum transversum, including the liver, as illustrated by Fig. 259. It will be remembered, see p. 480, that the coelom extends above the septum transversum on either side of the CBSophagus, making during earl^' stages a free communication between the abdominal and pericardial cavities. In transverse sections one sees at once that the lungs project into the coelomatic passage (or future pleural cavity) above the liver and septum transversum. The manner in which the pleural cavities are finally shut oflf is described p. 482. It is essential to note that the lungs arise on the dorsal side of the heart and liver. The lungs (and pleural cavities) only gradually expand forwanl on the right and left of the heart.

In the above description heed is given only to the epithelial or entodermal portion of the pulmonary anlage. The epithelium is, however, surrounded by mesclerm, which makes a thick layer. In sections the rudimentary lung is readily seen to consist of a ring of epithelium composed of high cylinder cells ; the epithelium is inclosed by a thick layer of mesenchyma, and so far as the lung projects into the coelom it is, of course, covere<l by mesothelium. The mesothelium of the adult is known in descriptive anatomy as the epithelium of the pleural membrane.

Lungs. — The lungs arise, as described in the previous section, as two nearly symmetrical diverticula of the pulmonary anlage and immediately above the auricle of the heart (human embryo of 4 mm.). The diverticula lengthen out and grow dorsalward on either side of the oesophagus close to the cephalic end of the stomach, Fig. 444, B, and there form branches, C, all of which at first extend dorsalward. These branches of the entodermal diverticulum are inclosed in a thick covering of mesoderm; the two laj^ers thus associated constitute the embryonic lung. The organ as a whole projects into the coelom above the septum transversum ; its coelomatic surface is, of course, covered by mesothelium.

The branching entodermal tube forms the so-called bronchial tree, the entoderm itself persisting as the lining epithelium of the bronchi, bronchioles, infundibiila, and air-cells. The development of the branches during early stages has been traced by W. His, 87-3, in the human embryo, and less thoroughly by A. Robinson, 81.1, in rats and mice. The following account refers to man. The right diverticulum is somewhat larger than the left and extends further back — i)eculiarities which are, perhaps, connected with the changes a(!Companying the asymmetrical development of the heart. At six weeks the asymmetry of the lungs is more marked, for the right diverticulum is much longer and has three primar}^ branches budding forth, while the left lung has onl}' two ; each of these primary branches corresponds to a lobe of the adult lung, hence the right lung hiis three lobes, the left lung two. Morphologically, however, the upper and anterior branches of the right lung (and therefore the lol)cs they produce) are equivalent to the single upper branch of the left lung. Each branch elongates and branches, and the branches branch, and so on. Fig. 450 ; every branch is short and has a rounded and somewhat enlarged end ; as new branches are added those previously formed become stems and increase in diameter. The branching occurs in a highly characteristic maimer, for the stem always forks, but the forks develop unequally, one (terminal bud) growing more rapidly and becoming practic^ly the continuation of the main Btem, while the other (lateral bud) appears as a lateral branch. Speaking in general, it may be said that the ventral fork serves as the stem, c/. Fig. 44i, C, Lu. In consequence of this method of growth the adult lung consists of main stems with lateral branches, as we learned through the able investigations of Aeby ("DerBronchialbaum," etc., Leipzig, 1880). But it is

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suppose, as did Aeby, that the system of growth is strictly monopodia!, it being in realitj- a modified dichotomons system. The branches all arise by terminal forking, never as outgrowths from the side of a stem. In cross sections the limg has a triangular outline; one apex is the point of attachment and contains the main bronchus; the three sides we may designate as dursiil, lateral, and ventral; the branches of the bronchial tubes arrange themselves so that we can distinguish those toward the ventral from those toward the dorsal side, while the terminations of the tubes in the embrjo lie, for the moat part, toward the lateral side of the lung. Later the lungs revolve forward, and the ventral surface becomes mediid or cardial ; the lateral side corresponds to the costal surface. Fig. 4511 shows the bronchialramificationsof an embryo of 10.5 mm. ; they have been described in detail by His, I.e., '.<S. The same primary brandies appear in both lungs, and they occupy essentially symmetrical relations as r^^rds the veins; examined in detail, however, the two lungs are not perfectly symmetrical. The arteries, on the other hand, are entirely asymmetrical; the right nrteri*, A, Art, passes in front of, but the left artery' passes liehind, the first branch; this relation persists throughout life, an<l Itxl Aeby to designate the first right bronchfts as epaiterial and all the other bronchi hi/paiienal; Aeby — and His- seems to accept his view — inferred that the right lung contained a bronchus not represented in the left lung. I think, however, that this view is untenable and that the right and left Jirsi branches, I., are homologous ; the difference between the two sides is due to the precocious development on the right side, and to secondary modifications of the arteries; the relation of the veins to the bronchi confirms the interpretation here advanced. His' account of the development appears to me to flatly contradict Aeby's conclusion. The peculiar course of the right pulmonary artery is probably due to the abortion of the fifth aortic arch on the right side, and the consequent transfer of the origin of the artery to the left side ; if this suggestion is correct there should be in reptiles no eparterial bronchus.

The further ramifications of the bronchi begin as short, rounded buds forking off at the end of the branches, as may be easily seen in sections through the foetal lung, Fig.4fi2 ; hence the primary' branches are i)ennanent, and by their enlargement give rise to the main bronchi. Fig. 460 represents four views of the lungs of an embryo of five months, and is intended to show the homology of the two primary l6\yos on each side; the upper (and anterior) lobe of the right side, B, being partially subdivided.

Fig. 4G0.— Lungs of a Human Embryo of five Months Ofinot Coll., No. 71). A, From Ieft side; B, from right side; C, from l)ehin(l: 1>, from below.

Histogenesis. — The entodermal bronchial tul)es are at first widelv separated from one another ; the space between them is filled with mesonchyma. The tulx?s themselves have at first a high cylinder epithelium, Fig. 401, with oval granular nuclei, and have only a small lumen, but by their p^owth the mesoderm is condensed around them, forming a special envelojx^, Fig. 4(»1, which ultimately enters into the comiK)sition of the bronchial wall. The smooth muscle-fibres were observed in the bronchial wall in sheep embryos of 120 mm by L, Stieda, 78.1, 111. As development progresses the Tcunifica

tions of the bronchial tubes arise more rapidly than the growth of the mesenchyma, so that the amount of connective tissue between the branches gradually dimiDisbes, Fig 462, until at birth only thin partitions are left between the adjacent air-spaces. The epithelium remains in its embryonic stage ((.e., a high cylinder epithelium) in the bronchi in the bronchioles it becomes a cuboidal epithelium, in the infundibula (Alveolargaiige) and alveoli bonchi; later buds are and form bronchioles; ward the close of fix'I life the buds appear, hich are converted to the infundi bula and veoli, and these are ne<l with flattened epielium, as discovered sheep embr>-os of 250 m. by L. Stietla,77. 1, 3. The common asmption that the flat veoiar epithelium is >t present until the ng8 are stretched by e first breath is erro

Trachea.— Except on the early dcvelopent of the tnicliea as part of the pulmonary lage (p. 4G3) I have und no observations, sections of the traea of a four months' ibri-d, Fig. 4(i3, the ithelium is a high linder epithelium as early stages and also the adult, since the toderni i.if the trachea mglumt life; the cjiii;d with mucous cells.

opening into the cesophagus.

A six months' specimen (Minot Coll., No. 8) shows the glands well

advanced, as are also the tracheal cartilages.

Larynx. — The larynx is essentially the portion of the trachea best regarded as the metamorphosed pulmonary groove ; the groove early Incomes marked off from the cesophaguBor pharynx by two ridges, one on each side ; the ridges approach one another in front and devaricate posteriorly; they are the anlf^^ of the vocal chords. Kolliker found the anlages of the arj-tenoid cartilages the sixth week, but chondrification does not take place in the laiynx until the eighth or ninth week. The annular and arytenoid cartilages are disproportionately lai^ in early

iiteutiH>sefu(be«piibeiium.j period the larynx of the foetus is completely closed by the concrescence of its lining epithelium, a fact which was first recorded by Koth, 80.1, and since by Kolliker and Futelli, 88.1; Strazza, 88.1, ban published some oraervatious on the development of the laryngeal muscles. •

Epiglottis. — In a human embryo of 4.25 mm. His has found (•' Anat. menschlicher Embryonen," HI., GG) that the elongated opening of the larj'nx lies just behind the fourth branchial arch and is bounded bj' two slight ridges which meet in front, but fade out behind. He has also found that the ridges are the anlages of the median epiglottis and of the lateral ary-epiglottic folds. In an embryo of 10 mm.. Fig. 335, the epiglottis, Epg, is well developed. As to the further growth ami the histogenesis of the organ little is known, though a few details are given by Gangbofner, 80. 1.

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